An image sensor is a semiconductor device that converts a photo image, for example, light reflected by a subject, into an electric signal. Thus, image sensors are widely used in portable electronic devices, such as digital cameras, cellular phones, and the like. Generally, image sensors can be classified into charged coupled device (CCD) image sensors and complementary metal oxide semiconductor (CMOS) image sensors. Recently, CMOS image sensors have received more attention compared to CCD image sensors due to advantages, such as low manufacturing costs, low power consumption, ease of integration with peripheral circuits, and the like. Furthermore, CMOS image sensors may be classified into rolling shutter CMOS image sensors and global shutter CMOS image sensors. Studies have been performed on global shutter CMOS image sensors because, for example, image distortion, such as motion blur, jello effect, etc., of global shutter CMOS image sensors may be less than that of rolling shutter CMOS image sensors. Generally, a unit pixel included in the global shutter CMOS image sensor includes an overflow transistor that reduced the likelihood of overflow occurring in a photoelectric conversion device in a readout period. However, in a unit pixel included in conventional global shutter CMOS image sensors, image quality degradation may be caused because a leakage current due to hot carrier generation flows into the photoelectric conversion device, for example, a photodiode, etc., through the overflow transistor even when the overflow transistor is turned off in a photoelectric conversion period.